The invention relates to a polycrystalline ceramic composition usable for artificial prostheses in orthopedic and dental fields, and to a process for the preparation thereof.
Recently, it has been earnestly desired to develop new implant materials which can be safely buried in a human or animal body as a prosthesis for a bone or tooth damaged by an accident or by a disease such as bone tumor, dental caries or serious periodontic disease, and intimately bound to a vital tissue without any rejection phenomena so that the functions thereof are maintained for a long period of time.
Various metallic and organic materials have hitherto been used as an implant material for a bone or tooth in the orthopedic and dental fields. However, the metallic and organic materials have various drawbacks and, thus, are not satisfactory as a prosthesis in a human or animal body. On the other hand, ceramics of inorganic substances are stable in a human or animal body and have excellent affinity for a human or animal body, and therefore, they are becoming one of the most useful materials as a prosthesis in the orthopedic and dental fields. Recently, ceramics such as bioglasses of an SiO.sub.2 --Na.sub.2 O--CaO--P.sub.2 O.sub.5 type, vitreous carbons and Al.sub.2 O.sub.3 have been practically utilized as an implant material for a bone or tooth. However, these conventional artificial ceramic materials have drawbacks in that they are not bound chemically to a vital tissue and, thus, are only retained permanently in the body as an alien material.
Hydroxyapatite, which is represented by the formula Ca.sub.5 (PO.sub.4).sub.3 OH and is one of the calcium phosphates, constitutes a main component of the minerals of bones and teeth. Thus, for the last two or three years, ceramics based on hydroxyapatite have increasingly become of great interest in the orthopedic and dental fields. For example, publicly disclosed Japanese Patent Application No. 51-40400 discloses the use and production of a polycrystalline ceramic based on hydroxyapatite. According to this prior art, the polycrystalline ceramic is obtained by reacting a calcium ion and a phosphate ion in an aqueous medium, separating the resulting gelatinuous precipitate of calcium phosphate of a Ca/P ratio between 1.5 and 1.67 by mole from the aqueous medium and calcining the gelatinuous precipitate. In this art, however, it is troublesome to separate the gelatinuous calcium phosphate from the aqueous medium because the separation must be very carefully carried out.
The Ca/P ratio by mole of the gelatinuous calcium phosphate is changed with the lapse of time (see H. Aoki, Reports of the Institute for Medical & Dental Engeneering, No. 7, pages 107 through 111 (1973)). Accordingly, it is difficult to maintain the Ca/P ratio by mole of the gelatinuous calcium phosphate constant for a long time and, thus, it is impossible to preserve the gelatinuous calcium phosphate for a long time as a starting material which is always usable for the calcination. Further, if the hydroxyapatite obtained by heating the gelatinuous calcium phosphate from the aqueous solution reaction is heated to a temperature above 1,250.degree. C., the hydroxyapatite is partially decomposed into whitlockite. Thus, it is difficult to obtain a product having a constant composition.